The present invention relates to a control device for an electric machine. Furthermore, the present invention relates to a vehicle including a control device according to the present invention and a method for controlling an electric machine.
Electric machines are increasingly being integrated into the drive train of road vehicles. The electric machine may be used either in addition to an internal combustion engine, as in hybrid vehicles, or as a sole drive, as in electric vehicles. This implementation places high demands on the available continuous power and peak power, as well as on the efficiency, of the electric drive.
The electric drive is generally operated in torque control mode, i.e., the vehicle controller specifies a setpoint value for the torque which is to be supplied by the electric machine. This mode may be motor operation, i.e., driving, as well as generator operation, i.e., braking.
In such a conventional control system, the required setpoint torque is converted into an associated setpoint value pair for the components of the stator current in a field-oriented coordinate system, as a function of the operating point. These current setpoint values are then controlled via a current control, which calculates the required stator voltages.
For the coordinate transformation of the measured phase currents on the one hand and the supplied controller voltages on the other hand, the angle of the rotor field is also needed, which is generally obtained, for example, in the case of the permanent magnet synchronous machine (PMSM), via a measurement of the rotor angle.
The calculation of the setpoint current components generally takes place according to the following criteria:                minimizing the overall losses of the electric drive        minimizing the inverter losses        minimizing the machine losses.        
Since the level of the stator current is a dominant influencing factor both in the case of the inverter losses and the machine losses (for example, quadratic dependence in the case of ohmic losses), the setpoint current components are selected in such a way that a required torque is set having a minimal current magnitude. Thus, in the so-called base speed region, operating points approach the MTPC (maximum torque per current) characteristic curve, and in the field weakening region, the maximum available stator voltage is utilized in order to minimize the required field weakening current.
DE 10 2013 217 444 A1 shows an exemplary method for controlling an electric motor.